fix sim when no CMT col in data

[roninsightrx]

there was an edge that was not covered if

• an input dataset did not contain a CMT column
• a model with absorption compartment was used

This resulted in observations being generated with CMT=1 instead of 2, which led to errors. This fix appropriately checks if CMT is available in the dataset. If not, it will attempt to read from the ADVAN using luna::get_obs_compartment().